Book/Report

FZJ-2018-03929

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Errors of absolute methods of reactor neutron activation analysis caused by non-1/E epithermal neutron spectra: the influence of the error of alpha when using the zirconium double monitor method

Erdtmann, G. (Corresponding author)FZJ*

1992
Forschungszentrum Jülich GmbH Zentralbibliothek Verlag Jülich

Please use a persistent id in citations: http://hdl.handle.net/2128/19226

Report No.: Juel-2670

Abstract: Reactor neutron activation analysis is with most elements based on their (n, $\gamma$) -reactions with slow, i. e. with thermal and epithermal neutrons. The spectrum of the thermal neutrons equals approximately a Maxwellian distribution of the neutron velocity, according to the temperature of the irradiation position, whereas the epithermal neutron spectrum is approximately described by a 1/E-shape, i. e. the differential flux is inversely proportional to the neutron energy E. The fluxes and the ratio of the two components can be measured by several different methods. lt has been found, that the epithermal neutron spectrum of reactors usually deviates from the ideal 1/E-shape. lf these deviations are not considered, errors will occur with the flux measurements and with the results of activation analyses based on such flux measurements. In these cases, the epithermal flux spectrum is better described by 1/E$^{1+\alpha}$. The epithermal neutron spectrum shape factors, $\alpha$, are different in different reactors and depend also on the irradiation positions. They can be determined by several methods, but there remain some uncertainties with the true values of $\alpha$, caused by the errors inherent to the methods of its measurement and by variations in time and space occuring mainly with high flux reactors. These uncertainties of $\alpha$ gave rise to the assumption, that methods of activation analysis based on neutron flux determinations were unreliable and their results were not correct. Because the calculation of the error propagation with absolute methods of neutron activation analysis is somewhat complicated, it is seldom carried out and the influence of errors of $\alpha$ is often wrongly assessed. Therefore, it seemed useful to develop a computer program for the required calculations. In this report results are presented for absolute neutron activation analysis using the zirconium double flux monitor method. With this procedure the fluxes, $\Theta_{th}$ and $\Theta_{epi}$, are measured with zirconium wires, which are irradiated together with each set of samples. The $\alpha$-values of the irradiation facilities used were measured only once orthey are only occasionally checked, so that comparatively large uncertainties have to be considered. The errors of the analytical results as far as they are caused by errors of $\alpha$ are calculated by comparing results obtained with true $\alpha$-values with those obtained with erroneous $\alpha$-values. Additionally, the case of no $\alpha$-correction is investigated. The calculations are carried out for irradiation facilities at the reactors FRJ-2(Jülich), FRM(Garching) and HFR(Petten), which have different flux ratlos and $\alpha$-values and, therefore, the influence of uncertainties of $\alpha$ has different effects on the errors of the analytical results. With most reactions the error propagation factors are $\ll$ 1, so that errors of the results, as far as they are caused by errors of $\alpha$, are negligible compared to other, unavoidable uncertainties of procedures of trace element analysis. With some few reactions, however, for the calculation of appropriate corrections an accurate knowledge of $\alpha$ is necessary.

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Contributing Institute(s):

1. Publikationen vor 2000 (PRE-2000)

Research Program(s):

1. 899 - ohne Topic (POF3-899) (POF3-899)

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